Polymorphic variation is maintained through introducing new genes to populations, either by gene flow or by mutation. Differences between phenotypes in different populations can be explained by either natural selection; that is, increasing the frequency of advantageous phenotypes and reducing the number negative phenotypes, or by genetic drift – this acts randomly, and changes allele frequencies by chance, with no regard to phenotype. Knowing this, it’s expected that phenotype of Cepaea nemoralis will differ between habitats/populations due to drift and selection. Phenotype frequency can also be affected by persistent gene flow (movement of alleles by migration) from neighbouring populations. There would be a gradient of phenotype between neighbouring populations, if gene flow occurs between the two. The samples from this experiment were taken from two different habitats: low open/short scrub ground and high open/short scrub ground. If natural selection is acting on the different populations, the samples taken from similar habitats should have similar phenotypes and samples taken from dissimilar habitats should also have different phenotypes, however, if there appears to be no correlation between the habitats and the samples, genetic drift, gene flow or sampling error may be an explanation.
We are going on to study polymorphisms in Cepaea nemoralis, also known as the grove snail, which are an effective test subject as they ‘carry their genes on their shells’ – we can see the different genes present in the snails by looking at their phenotype – these phenotypes are clearly visible, and can be seen immediately, hence the effects of natural selection are clear – they also have a relatively low area of displacement which means they are highly available in the different microhabitats within the area being studied, are highly available, and Cepaea nemoralis are themselves highly polymorphic – making them perfect for the experiment. Polymorphism within the snail population can be manifested as a discontinuous genetic variation that results in different forms or types of individuals among the members of the same species. Polymorphisms can “promote” diversity within a population, it often persists over many generations because no single form has an overall advantage or disadvantage over the others regarding natural selection. Mutations can also be the cause for polymorphism within a population. Mutations can arise in the DNA level or they can be caused on a cellular level due to environmental factors such as exposure to sunlight.
This high level of polymorphism shows itself in the various banding patterns, and colouring of the snails -- ranging from pale cream, to dark brown. It is thought that the one of the ‘main’ reason for these polymorphisms is due to selection – in the form of camouflage; that is, to ensure the snails can effectively avoid predation. However, their colouring can also be due to environmental factors such as temperature -- darker shells heat up much faster than lighter snails, meaning they are more prone to drying out. We hypothesize that snails with darker shell colour (brown) will be found in the cooler, lower areas that are shady and covered by bushes and scrub – and hence by extension, snails with lighter shells (yellow, pink) will be found in the higher areas, that are more exposed, and warmer.
In previous experiments, the general trend was that geographic separation was not the main cause of species variation between testing sites. In this test a recent climate driven change explained most of the variance in genetic variation, reducing at the genetic variability, also known as a population bottleneck. Factors such as geographic barriers only played a minor role in variation. It is implied from other experiments that such characterisation in species distribution may be the norm for these kinds of invertebrates. The strong genetic drift evidenced in these studies suggest that there is a loss in genetic variation, and may diminish evolutionary potential.
In our own experiment, we will sample snail populations from at least 6 sites -- three sites for each of our two different location types; that is, high, exposed grassy areas or low, covered, areas with bushes. We use three sites for each condition so as to limit the effect of genetic drift and any possible population bottlenecks. From the averages of each site (after performing chi squared to ensure we can generalise, and each of the three sites is statistically similar). Furthermore, we will collect 50 samples from each of our sites (300 data points in total) and will use random sampling (counting the first 50 at each site to be found).
After conducting our experiment and analysing our results we can conclude there is a statistically significant difference in the number of yellow and brown morph snails at high and low altitudes and can hence reject the null hypothesis. The use of the chi squared test showed that there is little variation between each of our three sampling locations at each of the conditions and because of this we can assume any variation in morph frequency between the high and low locations is due to selection pressures of some kind. There were significantly more brown morph snails at low altitude, more covered areas and more yellow morph snails in high altitude, exposed, grassy areas. In shell banding there was insignificant variation between the three high altitude sampling locations but in low altitude there was significant variation in 0,1 and 2 banded snails between the three low altitude sampling locations. This could be the result of selective pressures or a population bottleneck. There were significantly more 4 and 5 banded morphs in all sampling locations than any of the other morphs (0,1,2 and 3 banded). In high altitude locations there were significantly more 3 banded snails than in low altitude locations (chi squared: 18.38>15.09).
The values also suggest that any differences in allele frequency are most likely to be due to genetic drift or natural selection, between the populations that are linked by gene flow, NOT due sample error, as this results in similar allele frequencies, meaning, no significant differences between the populations.The experiment was replicated three times – three different locations (covered/uncovered) were chosen, at both altitudes. The results were found to be repeatable, leading to the idea that differences between habitats are due to selection.
Various types of selection seem to be acting upon the snails : visual in the form of thrushes developing search images, resulting in only targeting specific coloured snails -- an experiment done on thrushes by Harvey et Al (1975) shows that when exposed to Cepaea nemoralis they prey upon one specific coloured shell at a time – that is, they may target pink only, and when all pink shells are exhausted, the thrush will move on to yellow shelled snails. Other forms of selection may be due to camouflage, or climatic selection, leading to differences in allele frequencies between populations. Genes that have greater fitness (based on different selection pressures) spread at a greater rate; which is why different populations have different allele frequencies, or particular alleles are more common. It can be argued that selection of this type should become fixed, and disadvantageous or deleterious alleles, should be wiped out, and disappear from the population. In practice, this does not always occur, as populations are often subdivided, hence polymorphs can be maintained for much longer. Also, in larger populations, fixation takes time to occur, so there is a long period of time when many polymorphisms are present.
However -- evidence in the form of precedent experiments shows that effects of allele frequencies can be unclear at higher altitudes – in the Marlborough downs, a uniform environment is present, but after a specific point, there is a sudden transition from light snails to dark snails ; unsure of why this change occurs so dramatically without a ‘smoothing’ effect, possible that this is due to limited or no breeding at the border between light or dark, or due to lack of genetic drift between populations, but this must be considered.
Polymorphic variation is maintained through introducing new genes to populations, either by gene flow or by mutation. Differences between phenotypes in different populations can be explained by either natural selection; that is, increasing the frequency of advantageous phenotypes and reducing the number negative phenotypes, or by genetic drift – this acts randomly, and changes allele frequencies by chance, with no regard to phenotype. Knowing this, it’s expected that phenotype of Cepaea nemoralis will differ between habitats/populations due to drift and selection. Phenotype frequency can also be affected by persistent gene flow (movement of alleles by migration) from neighbouring populations. There would be a gradient of phenotype between neighbouring populations, if gene flow occurs between the two. The samples from this experiment were taken from two different habitats: low open/short scrub ground and high open/short scrub ground. If natural selection is acting on the different populations, the samples taken from similar habitats should have similar phenotypes and samples taken from dissimilar habitats should also have different phenotypes, however, if there appears to be no correlation between the habitats and the samples, genetic drift, gene flow or sampling error may be an explanation.
We are going on to study polymorphisms in Cepaea nemoralis, also known as the grove snail, which are an effective test subject as they ‘carry their genes on their shells’ – we can see the different genes present in the snails by looking at their phenotype – these phenotypes are clearly visible, and can be seen immediately, hence the effects of natural selection are clear – they also have a relatively low area of displacement which means they are highly available in the different microhabitats within the area being studied, are highly available, and Cepaea nemoralis are themselves highly polymorphic – making them perfect for the experiment. Polymorphism within the snail population can be manifested as a discontinuous genetic variation that results in different forms or types of individuals among the members of the same species. Polymorphisms can “promote” diversity within a population, it often persists over many generations because no single form has an overall advantage or disadvantage over the others regarding natural selection. Mutations can also be the cause for polymorphism within a population. Mutations can arise in the DNA level or they can be caused on a cellular level due to environmental factors such as exposure to sunlight.
This high level of polymorphism shows itself in the various banding patterns, and colouring of the snails -- ranging from pale cream, to dark brown. It is thought that the one of the ‘main’ reason for these polymorphisms is due to selection – in the form of camouflage; that is, to ensure the snails can effectively avoid predation. However, their colouring can also be due to environmental factors such as temperature -- darker shells heat up much faster than lighter snails, meaning they are more prone to drying out. We hypothesize that snails with darker shell colour (brown) will be found in the cooler, lower areas that are shady and covered by bushes and scrub – and hence by extension, snails with lighter shells (yellow, pink) will be found in the higher areas, that are more exposed, and warmer.
In previous experiments, the general trend was that geographic separation was not the main cause of species variation between testing sites. In this test a recent climate driven change explained most of the variance in genetic variation, reducing at the genetic variability, also known as a population bottleneck. Factors such as geographic barriers only played a minor role in variation. It is implied from other experiments that such characterisation in species distribution may be the norm for these kinds of invertebrates. The strong genetic drift evidenced in these studies suggest that there is a loss in genetic variation, and may diminish evolutionary potential.
In our own experiment, we will sample snail populations from at least 6 sites -- three sites for each of our two different location types; that is, high, exposed grassy areas or low, covered, areas with bushes. We use three sites for each condition so as to limit the effect of genetic drift and any possible population bottlenecks. From the averages of each site (after performing chi squared to ensure we can generalise, and each of the three sites is statistically similar). Furthermore, we will collect 50 samples from each of our sites (300 data points in total) and will use random sampling (counting the first 50 at each site to be found).
Aaisha Gilani*, Nikita Singh, Netta Nyrhinen, Sashua Grinsted, Katie Bovey, Michael Selvasothy
Discussion
After conducting our experiment and analysing our results we can conclude there is a statistically significant difference in the number of yellow and brown morph snails at high and low altitudes and can hence reject the null hypothesis. The use of the chi squared test showed that there is little variation between each of our three sampling locations at each of the conditions and because of this we can assume any variation in morph frequency between the high and low locations is due to selection pressures of some kind. There were significantly more brown morph snails at low altitude, more covered areas and more yellow morph snails in high altitude, exposed, grassy areas. In shell banding there was insignificant variation between the three high altitude sampling locations but in low altitude there was significant variation in 0,1 and 2 banded snails between the three low altitude sampling locations. This could be the result of selective pressures or a population bottleneck. There were significantly more 4 and 5 banded morphs in all sampling locations than any of the other morphs (0,1,2 and 3 banded). In high altitude locations there were significantly more 3 banded snails than in low altitude locations (chi squared: 18.38>15.09).
The values also suggest that any differences in allele frequency are most likely to be due to genetic drift or natural selection, between the populations that are linked by gene flow, NOT due sample error, as this results in similar allele frequencies, meaning, no significant differences between the populations.The experiment was replicated three times – three different locations (covered/uncovered) were chosen, at both altitudes. The results were found to be repeatable, leading to the idea that differences between habitats are due to selection.
Various types of selection seem to be acting upon the snails : visual in the form of thrushes developing search images, resulting in only targeting specific coloured snails -- an experiment done on thrushes by Harvey et Al (1975) shows that when exposed to Cepaea nemoralis they prey upon one specific coloured shell at a time – that is, they may target pink only, and when all pink shells are exhausted, the thrush will move on to yellow shelled snails. Other forms of selection may be due to camouflage, or climatic selection, leading to differences in allele frequencies between populations. Genes that have greater fitness (based on different selection pressures) spread at a greater rate; which is why different populations have different allele frequencies, or particular alleles are more common. It can be argued that selection of this type should become fixed, and disadvantageous or deleterious alleles, should be wiped out, and disappear from the population. In practice, this does not always occur, as populations are often subdivided, hence polymorphs can be maintained for much longer. Also, in larger populations, fixation takes time to occur, so there is a long period of time when many polymorphisms are present.
However -- evidence in the form of precedent experiments shows that effects of allele frequencies can be unclear at higher altitudes – in the Marlborough downs, a uniform environment is present, but after a specific point, there is a sudden transition from light snails to dark snails ; unsure of why this change occurs so dramatically without a ‘smoothing’ effect, possible that this is due to limited or no breeding at the border between light or dark, or due to lack of genetic drift between populations, but this must be considered.
Word Count : 551
A.Gilani + group